As the number of users of wireless communication systems increases, the requirements on these systems for reliability and efficiency increase. Among other things, there are two conflicting requirements in designing a radio transmitter, especially when the envelope of the transmitted signal varies. These requirements are high linearity in the transmitter characteristics and high power efficiency. A power amplifier is one of the key components of the transmitter that determines the linearity and power efficiency of the transmitter. Usually, high power amplifier efficiency is obtained at the expense of reduced power amplifier linearity and vice versa.
In modern wireless communication systems, a spectrally efficient transmitted signal is preferable to support high data rate services using a given RF spectrum. If the transmitter characteristics are not linear enough, out-of-band emission can increase beyond the spectrum mask imposed by a standardization or regulatory body. Further, portable radio transceivers should be highly efficient in power consumption so as to provide extended operation time with a battery. A large portion of the total power available is consumed by the power amplifier. Therefore, it is important to use a power efficient power amplifier to conserve battery power. Linearization techniques may be considered as a possible solution to overcome tightened spectral mask requirements with acceptable power amplifier efficiency.
There are several well known approaches available for linearization of nonlinear power amplifier characteristics. These approaches may be referred to as feedforward, feedback, envelope elimination and restoration, and linear amplification with nonlinear components. The first two approaches are better suited for analog implementations.
In the feedforward methodology, a distorted power amplifier output is compared with an original power amplifier input signal, and a resulting error signal is subtracted in the analog domain. Feedforward can, in theory, eliminate the intermodulation distortion, but the key problem of this scheme is the need of an ideal gain and phase match between the two signal paths. A fine tuning of analog components is necessary, and the power consumed by any additional analog components may be substantial. The feedforward methodology is quite complex, and the total efficiency is drained due to losses in main-path delay, couplers, and auxiliary amplifiers.
In typical feedback methods, the power amplifier output signal is down-converted to baseband and compared with an original modulating signal. The resulting error signal modulates the carrier so that power amplifier output signal is substantially free of distortion. Among various feedback techniques, cartesian feedback has been proven to work for wideband applications, and polar modulation feedback is more suitable for narrowband systems. A problem with some of these feedback methods is that the feedback path operates in parallel with the transmitter at all times and the complexity of these schemes is also quite high. Further, these well known feedback methodologies can suffer from instabilities.
In the envelope elimination and restoration methodology, the modulated signal is decomposed into an envelope signal and a phase-modulated signal. The power amplifier is driven by just the phase-modulated signal while the envelope component controls a direct voltage supply and/or a direct current supply. In the linear amplification with nonlinear components methodology, the modulated signal is decomposed into two constant magnitude phase modulated signals, and the two components are summed after driving their respective power amplifiers. In general, the previously mentioned techniques complicate the design, require multiple fine adjustments, and become less effective as device characteristics change with temperature and output power.
Accordingly, what is needed in the art is a linearization technique that overcomes the limitations of the prior art and is adaptive to changing conditions.